The Müllerian mimicry theory

Müllerian mimicry is a form of biological resemblance in which two or more poisonous, or unpleasant tasting, organisms exhibit similar warning systems. These organisms, that may or may not be closely related, mimic each other’s warning signals, such as the same brightly colored wing pattern, to their mutual benefit. Because a predator that has learned to avoid an organism with a given warning system will avoid all similar organisms, the resemblance between Müllerian mimics acts as a protective mechanism for participating organisms.

Researchers conjectured that these distasteful organisms might have been caused to resemble each other by their physical environment. Through a variety of technical approaches in comparative developmental genetics and the construction of genetic linkage maps that can identify chromosome regions and sometimes even specific genes that control phenotypic traits. Müllerian mimicry was first identified in tropical butterflies that shared colorful wing patterns.

Aposematism

Müllerian mimicry relies on aposematism, or warning signals. Dangerous organisms with these honest signals are avoided by predators, which quickly learn after a bad experience not to pursue the same unprofitable prey again. Learning is not actually necessary for animals which instinctively avoid certain prey; however, learning from experience is more common. The underlying concept with predators that learn is that the warning signal makes the harmful organism easier to remember than if it remained as well camouflaged as possible. Aposematism and camouflage are in this way opposing concepts, but this does not mean they are mutually exclusive. Many animals remain inconspicuous until threatened, then suddenly employ warning signals, such as startling eyespots, bright colors on their undersides or loud vocalizations. In this way, they enjoy the best of both strategies. These strategies may also be employed differentially throughout development. For instance, large white butterflies are aposematic as larvae, but are Müllerian mimics once they emerge from development as adult butterflies.

Selective advantage

Many different prey of the same predator could all employ their own warning signals, but this would make no sense for any party. If they could all agree on a common warning signal, the predator would have fewer detrimental experiences, and the prey would lose fewer individuals educating it. No such conference needs to take place, as a prey species that just so happens to look a little like an unprofitable species will be safer than its conspecifics, enabling natural selection to drive the prey species toward a single warning language. This can lead to the evolution of both Batesian and Müllerian mimicry, depending on whether the mimic is itself unprofitable to its predators, or just a free-rider. Multiple species can join the protective cooperative, expanding the mimicry ring. Müller thus provided an explanation for Bates” paradox; the mimicry was not, in his view, a case of exploitation by one species, but rather a mutualistic arrangement, though his mathematical model indicated a pronounced asymmetry.

Genetics

Some insight into the evolution of mimetic color mimicry in Lepidoptera, in particular, can be seen through the study of the Optix gene. The Optix gene is responsible for the Heliconius butterflies” signature red wing patterns that help it signal to predators that it is toxic. By sharing this coloration with other poisonous red-winged butterflies the predator may have pursued previously the Heliconius butterfly increases its chance of survival through association. By mapping the genome of many related species of Heliconius butterflies “show[s] that the cis-regulatory evolution of a single transcription factor can repeatedly drive the convergent evolution of complex color patterns in distantly related species…”. This suggests that the evolution of a non-coding piece of DNA that regulates the transcription of nearby genes can be the reason behind similar phenotypic coloration between distant species, making it hard to determine if the trait is homologous or simply the result of convergent evolution.

One proposed mechanism for Müllerian mimicry is the “two-step hypothesis”. This states that a large mutational leap initially establishes an approximate resemblance of the mimic to the model, both species already being aposematic. In a second step, smaller changes establish a closer resemblance. This is only likely to work, however, when a trait is governed by a single gene, and many coloration patterns are certainly controlled by multiple genesMüllerian mimicry, advergence may be more common than convergence. In advergent evolution, the mimicking species respond to predation by coming to resemble the model more and more closely. Any initial benefit is thus to the mimic, and there is no implied mutualism, as there would be with Müller’s original convergence theory.

However, once model and mimic have become closely similar, some degree of mutual protection becomes likely. This theory would predict that all mimicking species in an area should converge on a single pattern of coloration. This does not appear to happen in nature, however, as Heliconius butterflies form multiple Müllerian mimicry rings in a single geographical area. The finding implies that additional evolutionary forces are probably at work

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